1. Field of the Invention
This invention relates to a playback magnetic head and particularly to a magneto resistance effect type magnetic head having a thin film magneto-resistance effect element.
2. Description of the Prior Art
A magneto resistance effect type magnetic head is generally formed by using a material having a magneto-resistance effect (hereinafter simply referred to MR effect) such as a magnetic alloy thin film formed of a material such as Ni-Fe(Nickel - Iron) system alloy or Ni-Co (Nickel - Cobalt) system alloy. The alloy thin film having the MR effect is formed on a nonmagnetic substrate like, for example, a sapphire, having a predetermined width equivalent to a reproduced track width in which a film thickness is several hundreds .ANG. and a length is several tens to several hundreds .mu.m thereby composing a magnetic sensor portion of the MR head. In the MR type magnetic head, a contact surface thereof facing a magnetic recording medium is adapted to be formed by one end surface of the thin film MR element, and a direction of transport or scanning direction of the recording medium relatively to the magnetic head is adapted to correspond to a film thickness direction of the thin film MR element.
It is said in general that the playback magnetic head of the MR effect type can obtain a higher sensitivity upon reproducing a narrow track, a short wavelength and a reproduction with an ultra low speed as compared with a normal electro-magnetic induction type playback head.
However, in the MR effect type playback magnetic head of this sort, a response characteristic upon reproducing the short wavelength is not always improved sufficiently in practice. It may be considered that the reason is mainly because a blind portion exists in the vicinity of the contact portion of the thin film MR element with the magnetic recording medium so as to prevent the MR effect from being satisfactorily demonstrated, by which the response characteristic upon reproducing the short wavelength is lowered.
There is provided a thin film MR element 1 patterned like a band shape as illustrated in FIG. 1. If a current I flows in a longitudinal direction thereof and an uniform magnetic field H is applied thereto in a direction to intersect the direction of the current I as well as in a direction to extend along a film surface of the MR effect element 1, a resistance change .DELTA.92 of the MR effect element 1 relative to the applied magnetic field H, and accordingly an output .DELTA.e thereof shows a characteristic as represented by a curve 2 in FIG. 2, in other words, a symmetrical characteristic with respect to the magnetic field .+-.H. In this case, if a signal magnetic field denoted by reference numeral 3 in FIG. 2 is applied to the MR effect element 1 from the magnetic recording medium under the condition that a bias magnetic field is not applied thereto, an output signal designated by a reference numeral 4 in FIG. 2, a polarity of which can not be identified, is produced as an output of the MR effect element 1.
To cope with the above aspect, it is arranged that the MR type magnetic playback head of this kind is operated under the condition that the MR effect element is supplied with predetermined DC(Direct Current) bias magnetic field H.sub.B as shown in FIG. 3 so as to produce for an input signal magnetic field applied thereto and designated by a reference numeral 3' in FIG. 3 an output signal represented by reference numeral 4' in FIG. 3 in which the polarity of the input signal magnetic field can be discriminated is produced.
On the other hand, referring in particular to a distribution of a demagnetizing field in the aforesaid thin film MR effect element 1, a strength of the demagnetizing field with respect to the widthwise direction of the MR effect element 1 is lowered at the center of the widthwise direction of the MR effect element 1 but raised higher at both end surfaces 1a and 1b thereof as seen in FIG. 4. Accordingly, if a film width w of the MR effect element 1, i.e., a distance w between one end surface 1a forming a side closely facing to a magnetic recording medium 5 and the other end surface 1b forming another side to oppose to the end surface 1a is selected, as for example, 5 .mu.m, the film thickness of the MR effect element 1 is selected as 500 .ANG. and the length thereof is selected as 60 .mu.m and when the MR effect element 1 occupied by both end surfaces 1a and 1b is divided into regions as shown by respective broken lines a.sub.1 to a.sub. 5 in FIG. 5 and the respective divided regions are represented as the first to sixth regions counted from the end surface 1a side, the characteristics of the resistance changes in the respective regions relative to the bias magnetic field H.sub.B are different from one other as illustrated in FIG. 6. In other words, as shown in FIG. 6, a curve 6 is employed to indicate a characteristic in the first and sixth regions, a curve 7 a characteristic in the second and fifth regions and a curve 8 is provided to indicate a characteristic in the third and fourth regions. More specifically, in the central region of the film width of the MR effect element 1, i.e., the third and fourth regions, as will be clear from FIG. 6, particularly when the film thickness of the MR element 1 is thin so as to make the demagnetizing field small or weak and a magnetic anisotropic field H.sub.K is further given as small, a magnetization is apt to be saturated by a relatively small magnetic field, by which the resistance change is urged to be saturated. On the contrary, the strength of the demagnetizing field is enlarged by an effect of an end edge in the first and sixth regions at the respective end surfaces 1a and 1b so that the magnetization is difficult to be saturated, thereby allowing the resistance change characteristic to demonstrate a wide range of a linearity.
But, an overall output of the MR effect element 1 is determined by the respective characteristics over the first to sixth regions thereof and hence in the prior art, the bias magnetic field is preset at a substantially central value of the magnetic field range in which the resistance change characteristic thereof can have the linearity in the overall characteristic. That is, the bias magnetic field is preset or selected to be relatively low or small value such as 150 to 200 Oe. Referring to the curve 6 as seen in FIG. 6, i.e., the first and sixth regions forming the sides of both end surfaces 1a and 1b in the MR effect element 1, such low bias magnetic field is forced to become the region in which the resistance change .DELTA..rho. is quite small. Whereas, the more the signal magnetic field to be supplied to this MR effect element 1 becomes the short wavelength, the spacing loss becomes larger, whereby a proximity portion of the surface on the side to which the signal magnetic field is applied to the MR effect element 1, more specifically, a nearby region of the end surface 1a composing the contact surface side with the magnetic medium 5, i.e., only the first region as set forth above serves for reproduction. But, in this first region, the resistance change characteristic becomes the characteristic shown by the curve 6 in FIG. 6. Since the bias magnetic field is not properly selected with respect to the first region previously mentioned, it is apparent that the short wavelength characteristic of the MR effect type playback magnetic head of this kind is not sufficiently improved as already mentioned in the beginning.
Although an improvement of such short wavelength characteristic is realized to a certain extent by making the film width of the MR element, for example, small, it is impossible to form the film width thereof sufficiently small in a view point of obtaining a long life of the MR element, considering the abrasion thereof due to a sliding with the magnetic recording medium.